The author of the paper listed below writes about mushroom fairy rings of the species Melanoleuca brevipes in Ontario, Canada. The sites were near meandering streams where steep, eroding banks alternated with lower, gently sloping banks. The author would mark the fruiting body positions with distinctive sticks so as to be able to find the site of the mycelium in the future and note the annual growth rate. The annual expansion in the ring was between 9 and 12 inches (roughly 20 to 30 centimetres) and it became clear that this expansion would bring parts of some of the mycelial rings right to the eroding stream banks.
One day, while trout fishing in a stream he was forced to disentangle his line from a streamside shrub and noticed a single Melanoleuca brevipes mushroom right at the water's edge. At this point the bank was low, grassy and with no sign of erosion. It also appeared to be a spot where he had not previously noted the species. He placed a marker near the mushroom and re-checked the spot over three successive years. In the following year there were three mushrooms at the top of the bank. In the second year there were seven, in an arc leading away from the water and in the third year there were nine, with the arc a further eight inches away from the water.
The author noted that during times of high flow the currents would be strong enough to carry small soil clumps downstream. Therefore, if a mycelium laden clump falls into the water from an eroding bank it could be carried downstream and come to rest at a bend or be trapped by some streamside obstruction. If the place in which such a clump comes to rest is a suitable habitat for the fungus there would be nothing to stop the mycelium growing out from the clump and through the surrounding soil.
These observations are not definitive proof that a dislodged, mycelium-laden earth clod was responsible for that sequence of 1, 3, 7 and 9 mushrooms spreading away from the water's edge - but the evidence is certainly highly suggestive.